The invention relates to a method for coating shaped metal bodies first with an aqueous acidic oxalating solution and, if necessary, next with a lubricant composition in particular in the form of an aqueous solution or dispersion based on organic polymer(s)/copolymer(s), an oil emulsion, an oil, a solid lubricant or a dry lubricant such as soap powder to prepare the metallic shaped body for cold forming.
A cold forming may take place typically at surface temperatures up to about 450° C. without external heat supply. The heating can thus occur solely due to the frictional forces between the coated metallic shaped body blank and the tools in effect during the reshaping and due to internal frictional forces caused by material flow, however where necessary also by preheating the shaped body to be reshaped. However, usually the temperature of the shaped body to be reshaped begins often at environmental temperature, thus at approximately 10 to 32° C. But if the shaped body to be reshaped is pre-heated to temperatures in the range, for example, from 650 to 850° C., from 850 to 1250° C. or from 650 to 1250° C., medium temperature forming or forging occurs. In addition, during the cold forming step high pressure increase usually occurs, e.g. for steel in the range of 200 MPa to 1 GPa and partially even to 2 GPa.
As shaped bodies to be reshaped, most often strips, sheets, slugs, wires, wire coils, intricately formed shaped parts, sleeves, profiles such as hollow- or solid profiles, pipes, round blanks, discs, rods, bars and/or cylinders are used. A slug is a disc or a section of a wire, of a wire coil or of a bar.
The metallic shaped body to be cold-reshaped can basically consist of any metallic material. Preferably it is essentially composed of steel, aluminum, aluminum-alloy, copper, copper-alloy, magnesium-alloy, titanium, titanium-alloy, in particular structural steel, high-tensile steel, stainless steel, iron- or steel material with chrome content and/or metal-coated steel such as aluminized or galvanized steel. Most often the shaped body is substantially made of steel.
While forming oils are normally used in the cold forming of metallic shaped bodies having lower degrees of forming and corresponding lower strengths, at much higher degrees of forming as a rule at least one coating is used as separation layer between shaped body and tool in order to prevent a cold welding of shaped body and tool. In the latter case, it is conventional to provide the shaped body with at least one coating of a lubricant or with a lubricant composition to reduce the frictional resistance between the surface of the shaped body and the shaping tool.
The cold forming comprises above all:                a slip-type drawing (push-pull forming), for example of welded or seamless pipes, hollow profiles, solid sections, wires or bars such as in wire drawing or tube drawing, for example,        a flow turning, wall-ironing (reshaping to final dimension) and/or deep drawing, for example of strips or sheets into especially deep-drawn shaped bodies or of hollow bodies into strongly deformed hollow bodies,        a roll threading and/or thread tapping, e.g. in the case of nut- or screw blanks,        a pressing, such as e.g. cold extrusion (pressure forming), e.g. of hollow or solid bodies or extrusion methods, and/or        a cold forging, e.g. of wire sections into connecting elements, for example nut- or screw blanks.        
Earlier, metallic shaped bodies were almost only prepared for cold forming either by applying a fat, an oil or an oil emulsion. For a long time, a lubricating layer usually followed on a separating layer, to minimize the friction occurring during the reshaping. In so doing, the blanks usually are first coated with zinc phosphate to form a separating layer and then either with a soap in particular based on alkali- and/or alkaline earth stearate and/or with a solid lubricant in particular based on molybdenum sulfide and/or carbon to form a lubricating layer, before the thus-coated blanks are cold formed.
The aforementioned lubricating system of the state of the art is mainly built up on zinc phosphate as separating layer. Here, however, the conditions of environmental compatibility and industrial hygiene as well as the requirements for safety-relevant components of phosphate-free and of low-in-heavy-metals baths and coatings are considered much more strongly today than in the past.
The metallic shaped bodies to be cold formed are pre-coated prior to the cold forming. During the preparation for cold forming, the metallic surface of the shaped body or the metallically coated coating thereof can be provided with a conversion coating, in particular be oxalated or phosphated. The conversion coating can preferably take place using an aqueous composition based on oxalate, alkali phosphate, calcium phosphate, magnesium phosphate, manganese phosphate, zinc phosphate or related mixed crystal phosphates, such as e.g. ZnCa-phosphate. Sometimes the metallic shaped bodies are coatless, i.e. without a prior conversion coating, but wetted with a lubricant composition. However, the latter is only possible when the metallic surface of the shaped body to be reshaped is chemically and/or physically cleaned beforehand.
In the following, the steels usable according to the invention will be characterized as such, which have a carbon content in the range of 0 to 2.06 wt. % and thus are not part of the iron-materials, and as such, have a chrome content in the range of 0 to <10 wt. % and in particular in the range of 0.01 to 9 wt. %, of 0.05 to 8 wt. %, of 0.1 to 7, of 0.2 to 5 wt. %, of 0.25 to 4 wt. % or of 0.3 to 2.5 wt. %. These include on the one hand according to DIN EN 10025 so-called structural steel, unalloyed steel, unalloyed quality steel, unalloyed stainless steel, microalloyed steel, low alloy steel and high alloy steel, and on the other hand, also case-hardened steel according to DIN EN 10084 and heat-treatable steel according to DIN EN 10083. These steels are designated hereinafter as “usable according to the invention” or as “not corrosion-resistant” if they have within the scope of this invention a chrome content of less than 10 wt. %. In comparison to these steels, which basically are cold-formable, cast iron is nevertheless not cold-formable.
Steels have carbon content in the range of 0 to 2.06 wt. %. Of the various element contents of steel, the chrome content of the steel above all influences the pickling attack of an acidic aqueous oxalating composition and also of an acidic aqueous zinc phosphating composition. Then when the chrome content is clearly under 10 wt. %, a passivating layer forms on the steel, which protects the steel from oxidation and chemical attack. Thus, in this way the pickling attack on the substrate is hindered or completely inhibited, and a separating layer is not formed, because no iron can be etched out from the substrate.
In order to form separating layers on steels having a chrome content >10 wt. %, it is conventional to coat these components with an oxalating layer with the aid of an aqueous halogen- and thiosulphate-containing oxalating solution. The oxalating solution so activated makes possible a drastically higher pickling attack than an aqueous halogen- and thiosulphate-free oxalating solution. As has now been found, the oxalating solutions of the prior art lack a capability to reduce the pickling attack with sufficient formation at the same time of an adhesive oxalate layer. Therefore, up to now no steels having a carbon content in the range of 0 to 2.06 wt. % and a chrome content in the range of 0 to <10 wt. % could be coated with oxalate layers. Therefore, such steels were coated at greater expense and with more-polluting and altogether more expensive zinc phosphate coatings, while only steels having chrome content of <5 wt. % can be zinc phosphated.
A stronger pickling attack results overall in the coating of steel-blanks having a carbon content in the range of 0 to 2.06 wt. % and a chrome content in the range of 0 to <10 wt. % with aqueous oxalating containing e.g. thiosulphate and/or halogen compound, so that an inadequate, namely a too-thin and unclosed separating layer or even no separating layer at all is formed. These oxalate layers were completely unsuitable for a cold forming.